Cell lines have become a valuable tool for vaccine manufacturing. The production of some important vaccines and viral vectors is still done in embryonated chicken eggs or primary chicken embryo fibroblasts. Primary avian tissue for virus replication is provided by SPF (specific pathogen free) production plants. SPF derived tissues are expensive and the quality of the supply material is often hard to control. Therefore, inconsistency and shortage in supply are the most predominant disadvantages of the technologies based on SPF eggs. The same is true for approaches where primary fibroblast monolayer cultures are used. To multiply cell lines indefinitely, the cells need to be immortalized. Most immortalized cell lines currently in use are descendants of cancer cells or of fused hybridoma cells. However, the later technology is limited to fusion with myeloma cells. No general technology exists that can generate immortalized cells of different types.